Rewind targets only a spell. It doesn’t target any lands. The lands are chosen as Rewind resolves.

3/14/2017

You can choose up to four lands, no matter who controls any of them. Those lands untap once. You can’t choose one land and have it untap four times, for example.

3/14/2017

If the spell is an illegal target when Rewind tries to resolve, perhaps because it was countered by another spell or ability, Rewind will be countered and none of its effects will happen. You won’t untap any lands.

3/14/2017

If Rewind resolves but the target spell can’t be countered, you’ll still untap lands.